Plasma torch cutting device and process

ABSTRACT

A method of operating a plasma arc torch system includes placing a work piece to be cut on a table of the plasma arc torch system, wherein at least a portion of the work piece has a planer surface facing away from the table. A plasma arc torch is positioned adjacent the planer surface of the work piece using a positioning apparatus, wherein the positioning apparatus has at least five degrees of freedom about which it can move the plasma arc torch relative the work piece for cutting the work piece on the table. The method further includes angling the torch relative the planer surface of the work piece such that the torch is held at an angle of between about 1 and about 4 degrees from perpendicular with the planer surface to back burn a produced kerf such that a kerf edge is perpendicular relative the planer surface of the work piece. Additionally, the planer surface of the work piece is calculated by contacting the work piece with the torch at least three times.

CROSS REFERENCE TO RELATED APPLICATIONS

This application claims the priority benefit of U.S. Provisional PatentApplication Ser. No. 61/217,361 filed May 29, 2009 and U.S. ProvisionalPatent Application Ser. No. 61/275,524 filed Aug. 31, 2009.

BACKGROUND OF THE INVENTION

1. Field of Invention

The present invention relates generally to plasma arc torch cuttingmachines and, in particular, relates to a positioning apparatus forpositioning the torch.

2. Description of Related Art

Plasma arc torches are widely used for cutting metallic materials andcan be employed in automated systems for automatically processing a workpiece. The system may include the plasma arc torch, an associated powersupply, a positioning apparatus, and an associated computerized numericcontroller (CNC). At least one of the plasma arc torch and the workpiece may be mounted on the positioning apparatus, which providesrelative motion between the tip of the torch and the work piece todirect the plasma arc along a processing path.

The plasma arc torch generally includes a torch body, an electrodemounted within the body, passages for cooling and arc control fluids, aswirl ring to control the fluid flow patterns, a nozzle with a centralexit orifice, electrical connections, and a power supply. The torchproduces the plasma arc, which is a constricted ionized jet of a plasmagas with high temperature and high momentum. A shield may also beemployed on the tip of the torch to protect the nozzle and to provide ashield gas flow to the area proximate the plasma arc. Gases used in thetorch can be non-reactive (e.g., argon or nitrogen), or reactive (e.g.,oxygen or air).

The power supply provides the electrical current necessary to generatethe plasma arc. The power supply has one or more dc power modules toproduce a constant current for the torch. Typically, the current can beset to discreet values. The power supply has a microprocessor, whichregulates essentially all plasma system functions, including startsequence, CNC interface functions, gas and cut parameters, and shut offsequences. For example, the microprocessor can ramp-up or ramp-down theelectrical current. The main on and off switch of the power supply canbe controlled locally or remotely by the CNC. The power supply alsopowers a cooling system for cooling the torch.

The CNC communicates with the positioning apparatus to direct the motionof the torch to enable the work piece to be cut to a desired pattern.However, with some cut geometries, such as circles, prior artpositioning systems that control the torch only in mutually orthogonalX, Y, and Z-axes produce a kerf that ends up being uneven as a smallportion of the metal remains. Additionally, at times, it would bedesired to cut the work piece with a bevel shaped edge. Therefore, thereis a strong need to provide an improved positioning apparatus for aplasma torch system that enables the operator to better control thepositioning of the torch to compensate for the angle of the side of thekerf or to provide a selected bevel cut.

SUMMARY OF THE INVENTION

The invention is directed to an improved plasma arc torch system forcutting a work piece. The system includes a table for receiving the workpiece and a plasma arc torch for cutting the work piece. The system alsoincludes a positioning apparatus for controlling the position andorientation of the cutting torch with respect to the work piece W on thetable, wherein the positioning apparatus having five degrees of freedomabout which it can move the plasma arc torch relative the work piece. Inone embodiment, the plasma arc torch system has an overlaying gantrywith a traveling beam disposed so as to traverse the table and acarriage mounted on the traveling beam on which the plasma torch ismounted. The traveling beam is movable along a first axis which extendsin a longitudinal direction by an X-axis motor, and the carriage ismovable along a second axis by operation of a Y-axis motor, and thecarriage is movable by operation of mechanical slider connected to aZ-axis motor for movement in along a third axis which extends in avertical direction relative to the table to move the plasma torch to adesired position on the table to cut the work piece, wherein the first,second and third axes are mutually orthogonal. The system also includesa turret mounted to the carriage, wherein the turret permits the torchto rotate about two different axes so that it can be tilted at an anglerelative the mutually orthogonal first, second and third axes, whereinthe turret contains a first pivot mount having a boom extendingtherefrom, and a second pivot mount on a distal end of the boom, whereinthe plasma arc torch is mounted on the second pivot mount.

The invention is also directed to a method of operating a plasma arctorch system. The method includes placing a work piece to be cut on atable of the plasma arc torch system, wherein at least a portion of thework piece has a planer surface facing away from the table. A plasma arctorch is positioned adjacent the planer surface of the work piece usinga positioning apparatus, wherein the positioning apparatus has at leastfive degrees of freedom about which it can move the plasma arc torchrelative the work piece for cutting the work piece on the table. Themethod further includes angling the torch relative the planer surface ofthe work piece such that the torch is held at an angle of between about1 and about 4 degrees from perpendicular with the planer surface to backburn a produced kerf such that a kerf edge is perpendicular relative theplaner surface of the work piece. Additionally, the method includescalculating the planer surface of the work piece by contacting the workpiece with the torch at least three times.

These and other features and advantages of this invention are describedin, or are apparent from, the following detailed description of variousexemplary embodiments of the systems and methods according to thisinvention.

BRIEF DESCRIPTION OF THE DRAWINGS

The structure, operation, and advantages of the presently disclosedembodiment of the invention will become apparent when consideration ofthe following description taken in conjunction with the accompanyingdrawings wherein:

FIG. 1 is a perspective view of a plasma arc torch system in accordancewith an exemplary embodiment of the invention;

FIG. 2 is an enlarged perspective view of a portion of the plasma arctorch system of FIG. 1; and

FIG. 3 is an enlarged perspective view of a portion of the plasma arctorch system of FIG. 1.

Corresponding reference characters indicate corresponding partsthroughout the views of the drawings.

DETAILED DESCRIPTION OF EXEMPLARY EMBODIMENTS

The invention will now be described in the following detaileddescription with reference to the drawings, wherein preferredembodiments are described in detail to enable practice of the invention.Although the invention is described with reference to these specificpreferred embodiments, it will be understood that the invention is notlimited to these preferred embodiments. But to the contrary, theinvention includes numerous alternatives, modifications and equivalentsas will become apparent from consideration of the following detaileddescription.

Referring now to the drawings, FIG. 1 illustrates a plasma arc torchsystem 10 in accordance with an exemplary embodiment of the presentinvention. The invention will be described using the plasma arc torchsystem 10, however, one skilled in the art will understand that theinvention may also be used on other cutting systems such as oxyfuel gascutting systems or laser cutting systems without departing from thescope of the invention. The system 10 includes a generally planar table12 for receiving a work piece W, a plasma arc torch 14 and a positioningapparatus 16 for controlling the position and orientation of the cuttingtorch 14 with respect to the work piece W on the table 12. The torch 14may be of any suitable conventional or special design, such as a dualgas plasma cutting torch. Although a detailed illustration of the plasmaarc torch 14 is omitted herein, it should be noted that plasma torchesuse a superheated stream of ionized gas issued at high velocity to cut astock material and the high temperature discharge from the torch forcutting the work piece W is generally referred to as a “flame.” Anelectrode is connected, through a torch cable for supplying plasmacurrent, to one terminal (minus terminal) of a plasma power source unitand the other terminal (plus terminal) of the plasma power source unitis connected to the work piece W through a parent material cable. Thecircuit for the work piece W diverges and is then connected to a nozzlethrough a resister and an on-off switch. Attached to the leading end(serving as a plasma arc generating point) of the electrode is aheat-resistant insert made from a high melting point material (e.g.,hafnium, zirconium and alloys) which can withstand the high heat of theplasma arc. Plasma arc torches are well known in the industry and neednot be described in further detail herein. A control station 20 ismounted adjacent the table 12 to provide the positioning apparatus 16with position control. The control station contains a suitablecomputerized numeric controller (CNC) 21.

According to the invention, the positioning apparatus 16 has fivedegrees of freedom about which it can move the plasma arc torch 14relative the work piece W. The positioning apparatus 16 includes anoverlaying gantry 22 with a traveling beam 24 disposed so as to traversethe table 12. Disposed on the traveling beam 24 is a carriage 26 onwhich the plasma torch 14 is mounted. The traveling beam 24 is movableby operation of an X-axis motor 27 in the direction of the X-axis alongan X-axis rail 28, which extends in a longitudinal direction (i.e., theX-axis direction) of the table 12. The carriage 26 is movable byoperation of a Y-axis motor 30 in the direction of the Y-axis along aY-axis rail 32 disposed on the traveling beam 24. The carriage ismovable by operation of mechanical slider 33 connected to a Z-axis motor34 for movement in a vertical direction (i.e., the Z-axis direction)relative to the table 12. By controlling each motor 27, 30, 34, theplasma torch 14 is moved to a desired position on the table 12 to cutthe work piece W.

As can be better seen in FIGS. 2 and 3, a turret 40 is mounted to thecarriage 26. The turret 40 permits the torch 14 to rotate about twodifferent axes so that it can be tilted at an angle relative themutually orthogonal X, Y, and Z-axes. The turret contains a first pivotrotary drive 42, a boom 44 extending from the first rotary drive 42, anda second rotary drive 46 on a distal end 48 of the boom 44. The plasmaarc torch 14 and an exhaust shroud 50 at least partially encirclingdischarge tip 52 of the torch 14 are clamped in a tool holder 54extending from the second pivot mount 46. Torch gas supply lines (notshown) extend from a port 56 on the rear end of the torch 14. Desirably,the shroud 50 is movable along the body of the torch 14 to position theshroud 50 close to the work piece 50 without bumping into the work pieceW when the torch 14 is positioned at an angle such as for a beveled cut.For example, when the torch 14 is tipped at an angle relative the workpiece W, the shroud 50 is moved up along the torch 14 so that the shroud50 does not contact the work piece W. When the torch 14 is substantiallyvertical relative the plane of the work piece W, the shroud is loweredto maintain close proximity with the work piece W. Desirably, the shroud50 is connected to the torch 14 such that a pneumatic actuator (notshown) may be used to raise and lower the shroud 50.

A suitable first motor, such as a first servomotor 60 coupled to atiming belt and ring gear (not shown), selectively rotates the firstrotary drive 42 for positioning the torch 14. Desirably, the firstservomotor 60 rotates the first rotary drive 42 about an axis parallelto the X-axis. A similar second motor such as a second servomotor 62selectively rotates the second rotary drive 46 for positioning of thetorch 14. Desirably, the second servomotor 60 rotates the second rotarydrive 46 about an axis parallel to the Z-axis when the first mount is inan initial or home position. Accordingly, once an understanding of thepositioning apparatus 16 is had, it should be appreciated that the torch14 may be moved to cut a wide variety of shapes in work piece W,including circular, square, slotted and other holes. Importantly, thetorch 14 may be positioned to cut beveled edges and to compensate forthe angle of the side of the kerf by back burning during cutting of thework piece W or to provide a selected bevel cut where desired.

A flexible hose (not shown) is connected to a duct 70 on the exhaustshroud 50. The hose is connected to a vacuum source (not shown) suchthat the vacuum source creates a low-pressure area in the immediatevicinity of discharge tip 52 of torch 14. To actively collect and removefluid debris such as noxious gases and vaporized matter produced bytorch 14, the exhaust shroud 50 is used to communicate a low-pressurearea in the immediate vicinity of discharge tip 52.

In operation, a user places the work piece W on the cutting table 12 andmounts the plasma arc torch 14 on the positioning apparatus 16 toprovide relative motion between the tip 52 of the torch 14 and the workpiece W to direct the plasma arc along a processing path. Typically, theuser provides a start command to the CNC 21 to initiate the cuttingprocess. The CNC 21 communicates with the positioning apparatus 16 todirect the motion of the torch 14 to enable the work piece W to be cutto a desired pattern. The positioning apparatus 16 uses signals from theCNC 21 to direct the torch 14 along a desired cutting path. Positioninformation is returned from the positioning apparatus 16 to the CNC 21to allow the CNC 21 to operate interactively with the positioningapparatus 16 to obtain an accurate cut path. The CNC 21 also controlsthe height of the shroud 50 on the torch 14 by controlling the pneumaticactuator.

In one embodiment, the control station 20 contains a torch heightcontroller (THC) that provides an optimum voltage for a desired metalcutting process. As one skilled in the art will understand, there is adirect relationship between cut voltage and a standoff. The standoffrefers to the gap between the surface of the work piece W and the tip 52of the torch 14. The THC directs the mechanical slider 33 driven by thez-axis motor 34. An encoder provided inside the motor 34 is inelectrical communication with the CNC 21. The encoder provides locationinformation from the slider 33 back to the CNC 21. The torch 14 is inelectrical communication with a voltage feedback card provided insidethe control station 21 to provide voltage information to the CNC 21. TheCNC 21 uses the location information provided by the encoder, andvoltage information provided by the voltage feedback card, inconjunction with a desired work piece cut path programmed into the CNC21, to provide an input signal to the motor 34 to change the standoff.The use on an encoder and a voltage feedback card are known in the artand need not be discussed herein in further detail.

To start the cutting process, the THC and CNC 21 direct the torch 14until contact is made with a work piece W in three different places onthe surface of the work piece W. Each time the torch 14 contacts thework piece W, a signal is sent from the voltage feedback card to the CNC21 to indicate the position of the work piece W. The CNC 21 uses thethree signals to calculate the planer surface of the work piece W. Inalternate embodiments, any means for detecting the surface of the workpiece W three times to calculate the planer surface may be used withoutdeparting from the scope of the invention. After the surface of the workpiece W has been determined, the torch 14 is retracted to a pierceheight as determined by the CNC 21. After the pilot arc in the torch 14has transferred to a cutting arc, a signal is sent from the voltagefeedback card to the CNC 21 allowing the CNC 21 to control the motion ofthe torch 14.

According to one embodiment of the invention, the height of the torch 14is calibrated relative the work piece W with the work piece W on thetable 12. Initially, the torch 14 is place in a substantiallyperpendicular orientation to the plane of the work piece W. The torch 14is lowered with the z-axis motor 34 until contact with the work piece Wis detected by a known means such as be reading the current limit on theservomotor or Ohmic sensing between the torch and work piece W. Thetorch 14 is then raised a determined height above the work piece W. Thetorch is then rotated relative the work piece a first specified angle A1by rotating the first rotary drive 42 with the first servomotor 60. Thefirst angle A1 is desirably between about 20 and about 60 degrees fromvertical. In one embodiment, the first rotary drive 42 rotates the torch14 a first angle A1 of 45 degrees. The torch 14 is then lowered with thez-axis motor 34 until contact with the work piece W is detected. Aftercontact is made, the torch 14 is raised and rotated back throughvertical to a second specified angle A2 relative the work piece byrotating the first rotary drive 42 with the first servomotor 60. Thesecond angle A2 is desirably between about 20 and about 60 degrees fromvertical in the opposite direction from the first angle A1. In oneembodiment, the second angle A2 has the same magnitude as the firstangle A1 but is in the opposite direction. With the torch 14 positionedat the second angle A2, it is lowered until contact with the work pieceW is detected. The torch 14 is then raised and rotated back to asubstantially vertical condition.

Next, the torch is rotated relative the work piece a third specifiedangle A3 by rotating the second rotary drive 46 with the secondservomotor 62. The torch 14 is moved in a plane to the third angle A3that is orthogonal to the plane in which the torch 14 was moved whenpositioning at the first and second angles A1 and A2. The third angle A3is desirably between about 20 and about 60 degrees from vertical. In oneembodiment, the second rotary drive 42 rotates the torch 14 a thirdangle A3 of 45 degrees. The torch 14 is then lowered with the z-axismotor 34 until contact with the work piece W is detected. After contactis made, the torch 14 is raised and rotated back through vertical to afourth specified angle A4 relative the work piece by rotating the secondrotary drive 46 with the second servomotor 62. The fourth angle A4 isdesirably between about 20 and about 60 degrees from vertical in theopposite direction from the third angle A3. In one embodiment, thefourth angle A4 has the same magnitude as the third angle A3, but is inthe opposite direction. With the torch 14 positioned at the fourth angleA4, it is lowered until contact with the work piece W is detected.

With the height of the torch 14 above the work piece W detected in thevertical position and at the four angled positions A1-A4, the distancebetween the tip 52 of the torch 14 and the intersection point P of theaxis X1 through the center of the torch 14 and the axis X2 extendingfrom the center of the second rotary drive 46 is calculated. Thisdistance is used in the CNC 21 to accurately control the position andangle of the torch 14 during cutting operations. Secondly, thecalibration automatically adjusts the torch 14 to be perpendicular tothe work piece W.

In one embodiment of the invention, during operation of the cuttingsystem 10, the positioning apparatus 16 orients the torch 14 relativethe planer surface of the work piece W such that it has a small angle orinclination so as to compensate for uneven edge sides of the kerf thatwould otherwise be formed if the torch 14 were held orthogonal to theplane of the work piece W at the location being cut. Desirably, thepositioning apparatus 16 adjusts the first and second pivot mounts 42,46 on the turret 40 such that the torch 14 is held at an angle ofbetween about 1 and about 10 degrees, and more desirably between about 1and about 4 degrees from vertical to back burn the kerf to produce akerf edge that is perpendicular relative the planer surface of the workpiece W.

In one embodiment of the invention, the torch 14 has a coolingarrangement wherein cooling water is passed via hoses to one or morecoolant chambers of the torch 14. The torch 14 houses the necessaryelectrical, gas and coolant conduits. The cooling water is passed to thetorch 14 via an inlet hose, whereupon the coolant first circulates in awater space of the body portion of the torch and exits via an outlethose. In one embodiment of the invention, the shroud 50 is also cooledby the closed-loop cooling system of the torch or by a separate coolingsystem. As best seen in FIG. 2, a cooling fluid inlet hose (not shown)is attached to an inlet port 74 on the shroud 50. A cooling fluid outlethose (not shown) is attached to an outlet port 76 on the shroud 50. Thecooling fluid cools the exhaust shroud 50.

While the disclosure has been illustrated and described in typicalexemplary embodiments, it is not intended to be limited to the detailsshown, since various modifications and substitutions can be made withoutdeparting in any way from the spirit of the present disclosure. As such,further modifications and equivalents of the disclosure herein disclosedmay occur to persons skilled in the art using no more than routineexperimentation, and all such modifications and equivalents are believedto be within the scope of the disclosure as defined by the followingclaims.

1. A cutting system comprising: a table for receiving a work piece; aplasma arc torch for cutting the work piece on the table; and apositioning apparatus for controlling the position and orientation ofthe cutting torch with respect to the work piece W on the table, whereinthe positioning apparatus having five degrees of freedom about which itcan move the plasma arc torch relative the work piece.
 2. The cuttingsystem of claim 1 further comprising an overlaying gantry with atraveling beam disposed so as to traverse the table and a carriagemounted on the traveling beam on which the plasma torch is mounted. 3.The cutting system of claim 2 wherein the traveling beam is movablealong a first axis which extends in a longitudinal direction by anX-axis motor, and the carriage is movable along a second axis byoperation of a Y-axis motor, and the carriage is movable by operation ofmechanical slider connected to a Z-axis motor for movement in along athird axis which extends in a vertical direction relative to the tableto move the plasma torch to a desired position on the table to cut thework piece, wherein the first, second and third axes are mutuallyorthogonal.
 4. The cutting system of claim 3 further comprising a turretmounted to the carriage, wherein the turret permits the torch to rotateabout two different axes so that it can be tilted at an angle relativethe mutually orthogonal first, second and third axes, wherein the turretcontains a first pivot mount having a boom extending therefrom, and asecond pivot mount on a distal end of the boom, wherein the plasma arctorch is mounted on the second pivot mount.
 5. The cutting system ofclaim 4 wherein a first servo motor selectively rotates the first pivotmount and a second servo motor selectively rotates the second pivotmount for positioning of the torch, wherein the first servo motorrotates the first pivot mount about an axis parallel to the first axisand the second servo motor rotates the second mount about an axisparallel to the third axis when the first pivot mount is in its homeposition such that the torch may be positioned to compensate for theangle of the side of the kerf by back burning during cutting of the workpiece.
 6. The cutting system of claim 5 further comprising acomputerized numeric controller (CNC) controlling the positioningapparatus.
 7. The cutting system of claim 1 wherein the cutting systemis a plasma arc torch cutting system, an oxyfuel gas cutting system or alaser cutting system.
 8. A method of operating a cutting systemcomprising using a positioning system having five degrees of freedom tocontrol the position and inclination of the torch, the methodcomprising: placing a work piece to be cut on a table of the plasma arctorch system, wherein at least a portion of the work piece has a planersurface facing away from the table; positioning a plasma arc torchadjacent the planer surface of the work piece using a positioningapparatus, wherein the positioning apparatus has at least five degreesof freedom about which it can move the plasma arc torch relative thework piece for cutting the work piece on the table; and angling thetorch relative the planer surface of the work piece such that the torchis held at an angle of between about 1 and about 4 degrees fromperpendicular with the planer surface to back burn a produced kerf suchthat a kerf edge is perpendicular relative the planer surface of thework piece.
 9. The method of operating a plasma arc torch system ofclaim 7 further comprising calculating the planer surface of the workpiece by contacting the work piece with the torch at least three times.10. The method of operating a plasma arc torch system of claim 9 whereinthe each time the torch contacts the work piece, a signal is sent fromthe voltage feedback card to the CNC to indicate the position of thework piece.
 11. The method of operating a plasma arc torch system ofclaim 8 further comprising calibrating the cutting system by calculatingthe distance between the tip of the torch and the intersection point Pof an axis through the center of the torch and an axis extending fromthe center of the second rotary drive.